program to create a doubly link list

# include <iostream.h>
# include <conio.h>
# include <malloc.h>
# include <process.h>

int display_menu();

struct link_list
{
 int no;
 struct link_list *next;
 struct link_list *prev;
};
class dqueue
{
 link_list *list,*head;
 public:
    dqueue()
    {
     head=NULL;
    }
    void insert_first();
    void insert_last();
    void delete_first();
    void delete_last();
    void display();
};
void dqueue :: insert_first()
{
  link_list *newnode;
  newnode = new link_list;
  cout<<\"Enter Number :\";
  cin>>newnode->no;
  list=head;
  if(list==NULL)
  {
   head=newnode;
   newnode->next=NULL;
   newnode->prev=NULL;
   return;
  }
  newnode->next=list;
  newnode->prev=NULL;
  list->prev=newnode;
  head=newnode;
}

void dqueue :: insert_last()
{
 link_list *newnode;
 newnode= new link_list;
 cout<<\"Enter Number :\";
 cin>>newnode->no;





 list=head;
 if(list==NULL)
 {
  head=newnode;
  newnode->next=NULL;
  newnode->prev=NULL;
  return;
 }
 while(list->next!=NULL)
 {
  list=list->next;
 }
 list->next=newnode;
 newnode->prev=list;
 newnode->next=NULL;
}

void dqueue :: display()
{
 cout<<endl;
 link_list *tail;
 list=head;
 if(head==NULL)
 {
  cout<<\"Empty Queue !!!\";
  return;
 }
 cout<<endl;
 cout<<\"Forward Display ...\"<<endl;
 while(list!=NULL)
 {
  cout<<list->no<<\"\\t\";
  if(list->next==NULL)
  {
    tail=list;
  }
    list=list->next;
 }
 list=tail;
 cout<<endl<<\"Backward Display ...\"<<endl;
 while(list!=NULL)
 {
  cout<<list->no<<\"\\t\";
  list=list->prev;
 }
}

void dqueue :: delete_first()
{
  list=head;
  list->next->prev=NULL;
  head=list->next;
}

void dqueue :: delete_last()
{
  list=head;
  while(list->next->next!=NULL)
  {
   list=list->next;
  }
  list->next=NULL;
}







void main()
{
 dqueue dq1;
 clrscr();
 while(1)
 {
  switch(display_menu())
  {
   case 1 : dq1.insert_first();
        dq1.display();
        getch();
        break;
   case 2 : dq1.insert_last();
        dq1.display();
        getch();
        break;
   case 3 : dq1.delete_first();
        dq1.display();
        getch();
        break;
   case 4 : dq1.delete_last();
        dq1.display();
        getch();
        break;
   case 5 : dq1.display();
        getch();
        break;
   case 6 : exit(0);
  }
 }
}

int display_menu()
{
 int ch;
 clrscr();
 cout<<\"[ 1 ] : Insert at First\"<<endl;
 cout<<\"[ 2 ] : Insert at Last\"<<endl;
 cout<<\"[ 3 ] : Delete From First\"<<endl;
 cout<<\"[ 4 ] : Delete From Last\"<<endl;
 cout<<\"[ 5 ] : Display\"<<endl;
 cout<<\"[ 6 ] : Exit\"<<endl;
 cout<<\"Enter your choice :\";
 cin>>ch;
 return(ch);
}